Unfortunately, both He ii \(\) 1640 and \(\) 4686 are relatively weak, blended lines. This has deterred their use as a reddening indicator. He ii \(\) 1640 is blended with O iii] \(\) 1663 and the red wing of C iv \(\) 1549; He ii \(\) 4686 is blended with broad Fe ii \(\) 4570 emission that can often be strong enough to make He ii \(\) 4686 unrecognizable. He ii \(\) 4686 can also blend with the blue wing of the strong H\(\)  line. Stellar absorption lines from the host galaxy are a further complication for He ii \(\) 4686. A problem with measuring He ii lines (especially \(\) 1640) is that higher-ionization lines are progressively broader than lower-ionization lines [1]}, [2]}, [3]} because the BLR is strongly radially stratified by ionization (see [4]}). Because of this, He ii cannot be de-blended by assuming that a He ii line has the same profile as the contaminating lines.
